1. Field of the Invention
The present invention relates to a radiation detecting apparatus for detecting radiation such as an X-ray or a γ-ray, and more particularly, to a radiation detecting apparatus suitable for use in a medical image diagnosis apparatus, a non-destructive examination apparatus, an analysis apparatus using radiation, and the like.
2. Description of the Related Art
Imaging methods used in medical diagnostic imaging can be classified into general imaging for obtaining a still image and radiographic imaging for obtaining a moving image. Suitable imaging methods and apparatuses may be selected as required.
One known method of general imaging for obtaining a still image includes exposing a film of a screen-film system (hereinafter referred to as an S/F system) comprised of a combination of a fluorescent plate and a film, developing the film, and then fixing the resultant image. Another known method is computed radiography (CR) in which a radiogram is first recorded in the form of a latent image on a photostimulable phosphor plate, and then the photostimulable phosphor plate is scanned with a laser beam and output optical information is read using a sensor.
However, a problem in these methods is that they require complicated work flow to obtain a radiographic image. Another problem is that a digital image can only be obtained indirectly via processing, which requires a long time. That is, a digital image cannot be obtained in real time. Thus, there is less merit in employing the conventional methods described above, compared with digital imaging methods such as computer tomograph (CT) or magnetic resonance imaging (MRI) used in medical diagnosis.
On the other hand, in the radiography for obtaining a moving image, one known method is to use an electron tube as an image intensifier (II). However, this method needs a large-scale apparatus including the electron tube. Also, the field of view or the detection area is typically not large enough to meet the requirements in medical diagnostic imaging. Furthermore, an obtained moving image includes a large amount of crosstalk arising from a specific structure of the apparatus, and it is desirable to reduce crosstalk to obtain a clearer image.
On the other hand, recent advances in the liquid crystal Thin Film Transistor (TFT) technology and information infrastructure have made it possible to realize a flat panel detector (FPD) composed of a sensor array and a fluorescent substance for converting radiation into visible light, wherein the sensor array is made up of optical-to-electrical converters using non-single silicon crystal such as amorphous silicon (a-Si) and switching TFTs. This technique is expected to make it possible to realize large-area imaging in a fully digital form.
The FPD is capable of reading a radiographic image and displaying the image on a display in real time. Another advantage is that a digital image can be obtained directly, and data can be easily stored, processed, and transferred. Although characteristics such as sensitivity depend on imaging conditions, the characteristics are generally similar to or better than the characteristics obtained in the conventional S/F or CR imaging techniques.
FIG. 13 shows a known equivalent circuit of an FPD. In FIG. 13, reference numeral 101 denotes a photoelectric conversion element, 102 denotes a transfer TFT, 103 denotes a driving line for driving the transfer TFT, 104 denotes a signal line, 105 denotes a bias line, 106 denotes a signal processing circuit, 107 denotes a TFT driving circuit, and 108 denotes an A/D converter.
If radiation is incident on the photoelectric conversion element 101, the incident radiation is converted in wavelength into visible light by a fluorescent substance (not shown). The resultant converted light is then converted to an electric charge by the conversion element 101 and stored in the conversion element 101. Thereafter, the TFT driving circuit 107 drives the transfer TFT 102 via the TFT driving line so as to transfer the stored charge to the signal processing circuit 106 via the signal line 104. The charge is processed by the signal processing circuit 106 and then converted by the A/D converter 108 from analog form into digital form. The resultant digital signal is output.
An example of the device structure widely used for the FPD has been described above. As for the optical-to-electrical converter, various device structures such as a p-type layer/intrinsic layer/n-type layer photodiode (PIN PD) and a MIS-type optical-to-electrical converter similar to that employed in the present invention have been proposed.
FIG. 14 is a plan view showing one pixel in which a MIS-type optical-to-electrical converter is used. In FIG. 14, reference numeral 201 denotes a MIS-type optical-to-electrical converter, 202 denotes a transfer TFT, 203 denotes a driving line for driving the transfer TFT, 204 denotes a signal line, 205 denotes a sensor bias line, 211 denotes a gate electrode of a transfer TFT, 212 denotes source and drain electrodes (hereinafter, referred to simply as SD electrodes) of the transfer TFT, and 213 denotes a contact hole.
FIG. 15 is a cross-sectional view of one pixel including various devices shown in FIG. 14. In FIG. 15, reference numeral 301 denotes a glass substrate, 302 denotes a driving line for driving the transfer TFT, 303 denotes a lower electrode of the MIS-type optical-to-electrical converter, 304 denotes a gate electrode of the transfer TFT, 305 denotes a gate insulating film, 306 denotes an intrinsic a-Si film, 307 denotes a hole blocking layer, 308 denotes a bias line, 309 denotes SD electrodes of the transfer TFT, 310 denotes a signal line, 320 denotes a protective film, 321 denotes an organic resin layer, and 322 denotes a fluorescent substance layer.
As can be seen from FIGS. 14 and 15, the MIS-type optical-to-electrical converter and the transfer TFT have the same layer structure, and thus they can be produced using a simple production method which allows a high production yield and low production cost. Furthermore, the FPD constructed in the above-described manner performs well in various aspects, including sensitivity, and it has come to be used in general imaging applications instead of conventional S/F method and CR method apparatuses.
However, although the FPD has the advantage that a fully digital large-area image can be obtained and the FPD has come to be used widely in general imaging, the FPD according to the conventional technology does not have a high enough reading speed needed in radiographic imaging.
FIG. 16 shows an equivalent circuit of a one-bit portion of an FPD using MIS-type optical-to-electrical converters. In FIG. 16, reference symbol C1 denotes total equivalent capacitance of the MIS-type optical-to-electrical converter, C2 denotes parasitic capacitance associated with the signal line, Vs denotes a sensor bias voltage, Vr denotes a sensor reset voltage, SW1 denotes a switch for selecting Vs or Vr applied to the MIS-type optical-to-electrical converter, SW2 denotes a switch for turning on/off the transfer TFT, SW3 denotes a switch for resetting the signal line, and Vout denotes an output voltage.
When the switch SW1 is at the Vs position, the voltage Vs is applied as a bias voltage to the MIS-type optical-to-electrical converter such that the semiconductor layer of the MIS-type optical-to-electrical converter is depleted. In this state, if light converted via the fluorescent substance is incident on the semiconductor layer, a positive charge blocked by the hole blocking layer is accumulated into the a-Si layer, and a voltage difference Vt occurs. Thereafter, when the on-voltage is applied to the transfer TFT via the SW2, the voltage Vout is output. The output voltage Vout is read by a reading circuit (not shown). After that, the signal line is reset by the switch SW3, and reading is performed sequentially.
By sequentially turning on transfer TFTs on a line-by-line basis according to the driving scheme described above, one entire frame is read. Thereafter, the MIS-type optical-to-electrical converter is reset by applying the reset voltage Vr to it via the SW1, and the bias voltage Vs is again applied thereby causing the charge accumulation to start in the reading operation.
For example, when the FPD has pixels with a size of 160 μm disposed in a pixel area with a size of 43 cm×43 cm, the total equivalent capacitance C1 of the MIS-type optical-to-electrical converter is about 1 pf and the parasitic capacitance C2 is about 50 pf. In such an FPD, when the charge is transferred, about 2% of the charge remains in the capacitor C1 without being transferred because of the charge sharing effect. Thus, to obtain a high-quality image, it is necessary to perform the resetting operation described above.
More specifically, the resetting operation needs ten msec or a few ten msec for each frame, depending on the resetting condition. Therefore, when it is desired to take a radiographic image at a rate of 30 frames per second (FPS) or at a higher rate, it is required to perform reading and resetting on all lines of one frame within a period of 33 msec (30 FPS).
FIG. 17 is a diagram showing a conventional method of driving an FPD. In FIG. 17, reference symbol T1 denotes a period of time needed to read one line, T2 denotes a period of time needed to read all lines, T3 denotes a reset time, and T denotes a period of time needed to perform the entire process on one frame. In the case in which it takes 33 msec to perform the entire process on one frame as described above, if the reset time T3 is equal to 15 msec, then T2 becomes 18 msec. Therefore, if there are 1500 lines to be read, the period of time T1 available for reading one line becomes 12 μsec. If a radiation exposure time, that is, a sensor accumulation time is taken into account, the reading period T1 is further limited. Thus, it becomes necessary to increase the transfer capacity of the transfer TFT. However, to increase in the transfer capacity of the transfer TFT, it is necessary to increase the size of the transfer TFT at the cost of the aperture ratio, which causes various problems such as a reduction in sensitivity, degradation in image quality, and an increase in the amount of radiation necessary to generate an image.
That is, a trade-off is needed between the high image quality and the high speed at which the FPD is driven to obtain a moving image. In other words, at present, it is impossible to achieve a high-speed moving image having high quality.
In view of the above, U.S. Pat. No. 5,869,837 to Huang discloses a radiographic image forming system including resetting means for periodically resetting capacitively coupled radiation detection means. However, in this radiographic image forming system, a protective film of a reading switch is also used as an insulating film of a reset switch. Also the connection position of the reset switch disclosed does not necessarily allow the radiation detection means to be fully reset. Thus there is some room for improvement in the layer structure and in resetting of the radiation detection means.